Protein tyrosine phosphorylation is a ubiquitous form of post- translational modification in the nervous system, suggesting that it may be involved in the regulation of neuronal function. Recently, protein tyrosine phosphorylation has been shown to be important in the synapse formation and synaptic plasticity at the neuromuscular junction. The phosphorylation of proteins is regulated by a balance in activity of protein kinases and protein phosphatases. While many protein tyrosine kinases have been characterized, much less is known about the relevant protein tyrosine phosphatases. The determination of the role of protein tyrosine phosphatases in the regulation of synapse formation and synaptic plasticity at the neuromuscular junction is the major goal of this research proposal. Twelve different protein tyrosine phosphatases have been identified from the rat skeletal muscle based on the consensus sequence of previously characterized tyrosine phosphatases, including PTP1D, a cytoplasmic phosphatase containing two src homology domains and three novel protein tyrosine phosphatases. A RNA splice variant of PTP1D, PTP1Di, has also been isolated which contains a four amino acid insert in the catalytic domain. The insertion of the four amino acid residues down-regulates the phosphatase activity. The PTP1D/PTP1Di phosphatases are abundantly expressed in the brain and skeletal muscle. These characteristics of PTP1D/PTP1Di, in addition to the SH2 domains which are known to interact with tyrosine-phosphorylated proteins, make them extremely interesting candidates for the phosphatase(s) in the regulation of synapse formation and synaptic plasticity at the neuromuscular junction. To study further the role of PTP1D/PTP1Di, we plan to: l) determine the expression of PTP1D/PTP1Di and their phosphatase activity during development and after denervation in the skeletal muscle, 2) investigate the mechanisms of the regulation of phosphatase activity of PTP1D/PTP1Di, 3) identify and characterize the proteins that directly interact with PTP1D/PTP1Di, 4) study the role of PTP1D/PTP1Di in the aggregation of nAChR. The results of the proposed research should contribute not only to understanding of the roles of protein tyrosine phosphatases in synaptic transmission and plasticity, but also to a more general understanding of the regulatory mechanisms of protein tyrosine phosphatase activity and their physiological substrates. Such knowledge is critical for the understanding the basis of neurological disorders at the neuromuscular junction such as muscular dystrophy and myasthenia gravis.